Dual-injection frac systems in horizontal wells ///

Dual-injection frac systems include either a coiled or standard tubing string inside the casing and allow fluid injection through both the internal string and the casing-tubing annulus. In most industrial applications, the internal string is coiled tubing (CT), and that is what we will assume for the rest of this article. All the discussions presented here are equally true if the internal string is regular tubing.
Dual-injection frac systems are available for both openhole and cased-hole completions. The common practice is to pump the slurry through one conduit (tubing or annulus), while maintaining a low injection rate through the other (usually referred to as the “dead” string). However, both strings are always available for pumping and are often used to address special situations.